I want to use SolarFast on silk. My question is, does the SolarFast material have to be washed in hot water in order for it to become permanent? This would be a problem, because silk cannot go into hot water, lukewarm maybe, but not hot.

You can use cool/warm water to rinse the dye from the material after development but you may end up with less development in your images. The hot water assist final development of the image and also is better at washing out excess dye. I have, over the years, discovered that silk is a very tough fabric and will actually handle hot water with little or no adverse affects.Another thing to note: because silk LOVES dye it is inclined to pick up some of the excess dye during the washing step even when you use the SolarFast wash and hot water. The result is that you will most likely end up with a gentle hue of the color you are using over the entire piece. We've come up with some beautiful pieces, but if what you are after are sharp white vs. color contrasts you will most likely find that difficult to achieve.